What chemicals were being mixed and the nature of the incident was being investigated by local fire officials, Chrysler spokeswoman Jodi Tinson said in a statement.

The incident occurred outside an old body shop produced noxious fumes.

Tinson said no one was injured or sickened during the incident. An emergency response plan was implemented and workers evacuated the plant while ventilation operations proceeded.

Area fire departments responded with mobile ventilation units and at least two ambulances.

Starting about 1:30 p.m. the parking lot outside the Chrysler plant began to empty as some employees were sent home.

The Belvidere Fire Department is investigating the cause of the incident with assistance of Boone County Rural Fire Protection District 2 and the Rockford Fire Department. The Illinois Environmental Protection Agency also was contacted, Belvidere Fire Chief Dave Worrell said.

Some Chrysler employees, with clearance from fire personnel, were able to return to work in some portions of the plant where air quality samples showed it was safe to do so soon after the incident.

A second shift reported to work at 6 p.m. as scheduled, Tinson said. But Belvidere firefighters did not leave the scene until about 7:20 p.m. It was not immediately clear when exactly the plant resumed operations.